1. Field of the Invention
The present invention relates generally to methods for aiding in the detection of cancer by utilizing computer analysis of radiologic images to identify spots corresponding to objects associated with malignancy. In its more particular respects, it relates to methods of aiding in the detection of breast cancer by computer identification of microcalcifications and clusters thereof in mammographic images.
2. Description of the Related Art
Breast cancer is one of the primary causes of death for women in western societies. Because the vast majority of deaths due to cancer that originated in the breast could be prevented by early detection, several national health organizations in the United States now recommend that all women over the age of 40 have regular screening mammograms. In fact, the National Cancer Institute has set a goal of 80% compliance by the end of the decade. To reach this goal, the number of mammograms taken and read in the U.S. would have to double, and in some localities, more than triple. An increase of this magnitude would overload the current capacity to take and interpret mammograms.
The reading or interpretation of screening mammograms is an art that requires extensive experience and attentiveness to detail. While the radiologist's primary sign for cancer is a mass visible on the mammogram, one of the more sensitive signs is the presence of small relatively bright spots (in film-screen mammography) corresponding to locally increased X-ray attenuation due to minute (&lt;5 mm in maximum dimension in the film image) deposits of calcium salts known as microcalcifications, each generally of irregular shape, and which are arranged in clusters. In fact, clustered microcalcifications are often the only sign indicating an early in situ malignancy. Unfortunately, these small spots generally appear in the images as obscured by gradations in background produced by surrounding tissue and their visibility is limited by the image's resolution, contrast and signal to noise ratio. Consequently, the probability of their detection by an experienced radiologist is not as high as desirable, making double independent readings a common practice to achieve acceptable results.
A method of computer identification of microcalcifications in digitized mammogram images is known from U.S. Pat. No. 4,907,156, which method is also applicable to a related problem of detection of spots in digitized chest X-ray images corresponding to lung nodules. Therein, spatial filtering was employed to deemphasize both low and very high spatial frequencies in the image attempting to increase the conspicuity of the microcalcifications. In particular, a difference image was formed of the results of application to the original image of a filter matched to a particular spot size and a median filter. An adaptive thresholding technique was used to label spots as calcifications, which spots also had to meet defined shape characteristics.
Accurate measurements of area, perimeter and shape of a spot are critical in classifying whether or not it corresponds to a microcalcification of a type associated with malignancy. Yet, prior art methods applied to make spots more conspicuous in radiologic images, such as that above-described, generally fail to preserve the locations of the edges or boundaries of the spots. Global spatial filtering with, for example, a Gaussian shaped filter, tends to smear these edges.
The problem of preservation of size and shape was addressed in J. Dengler et al. "Segmentation of Microcalcifications in Mammograms", (citation presently unknown). Therein, Gaussian high pass spatial filtering was used to remove the low frequency structural noise and "Difference of Gaussian" (DOG) spatial filtering was used to locate bright spots within a small range of sizes. Because this processing smoothed the boundaries of the spots, a morphological filter known as a "top-hat" transformation was applied to the original image to attempt to extract the boundaries of the spots. The spots located by the DoG filtering were iteratively expanded in size by topologically unimportant pixels, although not beyond the spot boundaries given by the morphological filter operation.
The efficacy of the method of J. Dengler et al. is dependent on proper choices for various spatial filter parameters (including positive and negative kernel widths, weight and threshold for the DoG filtering) which generally determine a range of sizes of spots that are detected as microcalcifications. Because of the irregular and frequently elongated nature of the microcalcification shapes of interest, it is not believed advisable to employ a size selective spatial filter which significantly restricts the range of spot sizes detected, particularly in a manner which is substantially independent of their shape. Further it does not seem wise to select candidate microcalcifications by thresholding intensity values. Such criteria may fail to select relatively dull spots which nonetheless have sharp edges.
Edge extraction is important in the general field of computer vision. An algorithm is discussed in P. Saint-Marc et al., "Adaptive Smoothing: A General Tool for Early Vision" IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol. 13, No. 6, pp. 514-529, 1991, which tends to maintain edge definition in filtered range images.